Z-Purlin Bending Check 1

[BadgerPE]

When checking purlins for an existing building, my bending check in RISA=2.643. However, when I check it by hand using the AISI equation that RISA used, I get 1.63 which is the square root of 2.643. It appears that the RISA calc missed the root. Has anyone had this problem? I should note that we are using RISA 3-D v5.0. We are looking to upgrade soon but I'm thinking this might have been corrected already.

Thanks all!

 

[JoshPlumSE]

A couple of things:

1) What cold formed code equations are you referencing? That older version of RISA was based (I believe) on the 1996 AISI code, but with the 1999 supplement.

I believe that the 1999 supplement changed some of those code check equations.

2) This is not an issue I could find in the RISA release notes as having been fixed. Granted that's going back almost 10 years now. And, I could only review the release notes for the last 5 years or so. So, it is a strong possibility (if it actually a bug) that it got fixed prior to that.

If you posted the file, I could let you know.

 

[BadgerPE]

Thanks for the quick response Josh. I was hoping you were patrolling the boards today. I have been using the 2007 AISI code which may be where my problem is coming from. I used the interaction equation C3.3.2-1 which RISA references but as I said before my answer was the perfect square of the bending check provided by RISA. Because of this, I thought there was a software glitch and not a code issue. Maybe I was wrong though. I have attached the file for your review. I really appreciate the help!

Jake

 

[JoshPlumSE]

The code check is the same in the current 8.1 (soon to be 9.0) version of RISA. But, there is a good explanation.

There is a minor hole in the AISI code checking procedure. At least, it's a hole if you want to say that one of te combined stress equations governed.

RISA always gives a combined stress equation as the governing equation. But, in this case, there is only on one moment in the member at the controlling location. The maximum moment occurs at the mid span where shear force is equal to zero. There is no weak axis moment, there is not torque, there is not axial load.

Therefore, the most correct value for the maximum code check at that location would be a simple Mu / PhiMn, right? Well, that's what RISA is reporting.

The only problem is figuring out what equation to list as the governing one. In their combined stress equations AISI always squares that bending term or puts a 0.6 multipler in front of it. So, those would technically result in a slightly lower code check.... but, a code check which would be misleading.

Therefore, RISA prefers to present the more accurate code check.